1. Field of Invention
The invention relates to a sheet feeder in which individual sheets are separately fed by a sheet feed roller.
2. Description of Related Art
A sheet feeder that feeds sheets to a printer, such as an ink-jet printer and a laser printer, typically includes a sheet feed cassette for storing a stack of sheets to be printed, a vertically movable pressure plate disposed at the bottom of the sheet feed cassette to support thereon the sheets, a sheet feed roller for feeding the sheets from the pressure plate, a push plate for moving up the pressure plate and bringing the sheets on the pressure plate into contact with the sheet feed roller, and a separation pad for separating a single sheet from the stack in contact with the sheet feed roller.
In a printer having such a sheet feeder, when a single sheet is separated from the stack, the pressure plate is moved up by the push plate toward the sheet feed roller and the stack on the pressure plate is brought into contact with the sheet feed roller with an optimum pressure. After that, when the sheet feed roller rotates, a single sheet is separated from the stack by the sheet feed roller and the separation pad.
On the other hand, when the separated sheet is conveyed, the push plate is lowered toward the bottom of the sheet feed cassette, and the pressure plate is also lowered to the bottom of sheet feed cassette. When the pressure plate is lowered, the stack on the pressure plate is apart from the sheet feed roller. This prevents the remaining sheets from being pressed against the sheet feed roller by the pressure plate. Accordingly, the next sheet beneath the separated sheet is prevented from being conveyed together with the seperated sheet. In addition, the rotation load of the sheet feed roller is reduced.
After that, when a sheet feed command is issued from a controller, the pressure plate is moved up by the push plate from the bottom of the sheet feed cassette toward the sheet feed roller, and the next sheet stacked on the pressure plate is brought into contact with the sheet feed roller. Then, the next sheet starts being conveyed.
In the above-described sheet feeder, when the sheet separated by the sheet feed roller and the separation pad is conveyed, the pressure plate should be lowered to the bottom of the sheet feed cassette by lowering the push plate from the upper position where the stack is in contact with the sheet feed roller to the lower position where the stack is away from the sheet feed roller.
After that, when the next sheet is separated, the push plate should be raised again from the lower position to the upper position to bring the next sheet stacked on the pressure plate into contact with the sheet feed roller.
In this way, when the pressure plate is moved up from the bottom of the sheet feed cassette toward the sheet feed roller by moving up the push plate from the lower position to the upper position, the moving distance of the pressure plate is so long that high impact noise is generated upon the contact between the sheets stacked on the pressure plate and the sheet feed roller. As a result, considerable noise is produced during the operation of the sheet feeder.
In order to separately feed a single sheet, the push plate that pushes the pressure plate should be vertically moved from the position where the sheets are in contact with the sheet feed roller and the position where the sheets are away from the sheet feed roller. Thus, another problem arises in that it takes a long time to separately feed a single sheet and, as a result, the sheet feed speed of the sheet feeder becomes low. To overcome this problem and to move the push plate at high speed, use of a high torque drive motor is conceivable. However, such a motor is expensive and results in an increase in the total production cost of the sheet feeder.
In view of the foregoing problems, one aspect of the invention is to provide a sheet feeder that generates low impact noise when a sheet support member is vertically moved by a pushing member, uses a low-powered lowering mechanism to lower the pushing member, and can reduce the sheet feed time per sheet to increase the sheet feed speed.
A sheet feeder according to the invention may include a sheet support member that supports thereon a stack of sheets, a sheet feed roller that is rotatably disposed above one end of the sheet support member and separates a single sheet from the stack of sheets and then conveys the separated single sheet, a pushing member that is vertically movable and pushes the sheet support member toward the sheet feed roller between a lower position where the stack of sheets is away from the sheet feed roller and an upper position where the stack of sheets is in contact with the sheet feed roller, an urging member that urges the pushing member from the lower position to the upper position, a lowering device that lowers the pushing member urged by the urging member by a predetermined distance from the upper position when the sheet feed roller conveys the separated single sheet, a halting device that halts the pushing member lowered by the lowering device at a position lower than the upper position and higher than the lower position, and a halt release device that releases the halted pushing member.
In the sheet feeder as structured above, a stack of sheets is put on the sheet support member when the pushing member is at the lower position. Thus, the stack on the sheet support member is away from the sheet feed roller. After that, the pushing member is urged from the lower position to the upper position and moved up toward the upper position. When the pushing member reaches the upper position, the stack on the sheet support member is brought into contact with the sheet feed roller. Then, a single sheet is separated from the stack in contact with the sheet feed roller as the sheet feed roller rotates.
After the single sheet is separated, the pushing member is lowered by the predetermined distance by the lowering device and halted by the halting device at a position lower than the upper position and higher than the lower position. The stack on the sheet support member falls away by the predetermined distance from the sheet feed roller. Thus, the separated sheet is conveyed by the sheet feed roller without being pressed by the sheet support member pushed by the pushing member against the sheet feed roller.
After the sheet is conveyed by the sheet feed roller, the halt release device releases the pushing device halted by the halting device. The pushing device is urged from the halted position toward the upper position by an urging force of the urging member. Thereby, the pushing member returns again to the upper position, and the sheet support member is pushed up toward the sheet feed roller. The next sheet of the stack is brought into contact with the sheet feed roller so as to be separated and conveyed.
The lowering device of the sheet feeder may include a ratchet gear having, on its outer circumference, a plurality of engaging pawls and rotated in a forward direction during upward movement of the pushing member toward the upper position and rotated in a reverse direction, which is opposite to the forward direction, during downward movement of the pushing member toward the lower position, a first cam member rotated as the sheet feed roller rotates, an actuating member projecting toward the first cam member and pushed by the first cam member as the sheet feed roller rotates, and an engaging member that, when the actuating member is pushed by the first cam member, engages one of the engaging pawls and swings in a same direction as the reverse rotation direction of the ratchet gear so as to rotate the ratchet gear by a predetermined angle in the reverse direction.
In the lowering device as structured above, the first cam member is rotated as the sheet feed roller rotates. The actuating member is moved by the rotation of the first cam member toward a certain direction such that the engaging member engages with one of the engaging pawls of the ratchet gear. When the actuating member is further moved by the first cam member, the engaging member swings in the same direction as the reverse rotation direction of the ratchet gear. Simultaneously, the ratchet gear is rotated in the reverse direction. Thereby, the pushing member is lowered by the predetermined distance toward the lower position.
Further, the first cam member is structured to release the actuating member when the ratchet gear has been rotated by the predetermined angle in the reverse direction and, upon release of the actuating member by the first cam member, the engaging member is released by one of the engaging pawls.
When the ratchet gear has been rotated by the engaging member by a predetermined angle, the actuating member is released by the first cam member, and thereby the engaging member is disengaged from one of the engaging pawls of the ratchet gear. This disengagement prevents the ratchet gear from rotating excessively in the reverse direction and the pushing member from being lowered more than the predetermined distance.
Further, the halting device of the sheet feeder may include a locking member that is engageable with one of the engaging pawls and, when engaged with one of the engaging pawls, restricts the forward rotation of the ratchet gear while permitting the reverse rotation of the ratchet gear.
Accordingly, the engagement of the locking member with one of the engaging pawls restricts the forward rotation of the ratchet gear. This restriction prevents the pushing member from moving up and halts the pushing member at the position lowered by the predetermined distance from the upper position. In addition, the locking member, when engaged with one of the engaging pawls, permits the reverse rotation of the ratchet gear. Thus, the locking member allows the pushing member to be lowered by the reverse rotation of the ratchet gear, while restricting the forward rotation of the ratchet gear and preventing the upward movement of the pushing member.
Further, the halt release device of the sheet feeder may include a second cam member that is rotated as the sheet feed roller rotates and pushes the locking member so as to release engagement between the locking member and one of the engaging pawls.
Accordingly, the engaging member, when pushed down by the second cam member, is disengaged from one of the engaging pawls. This disengagement allows the ratchet gear to rotate in the forward direction and the pushing member to move up toward the upper position. The pushing member is urged toward the upper position. The ratchet gear is rotated in the forward direction by this urging force and, in response to the rotation, the pushing member returns to the upper position.
The sheet feeder may further include an attenuator that attenuates the moving speed of the pushing member when the pushing member is moved upward by an urging force of the urging member.
Accordingly, impact of a stack of sheets on the sheet support member with the sheet feed roller is reduced. Because the stack on the sheet support member is prevented from colliding violently with the sheet feed roller, impact noise generated during upward movement of the push plate 24 is reduced.
The sheet feeder may further include a case and a sheet feed cassette detachably attached to the case and provided with the sheet support member. The sheet feed cassette may include an urging force starting member that makes the urging member start generating an urging force.
When the sheet feed cassette is attached to the case, the urging force starting member makes the urging member start generating an urging force. The generated urging force makes the pushing member move up toward the upper position.
Still further, the urging member of the sheet feeder may be made of an elastic material.
Accordingly, no electric device, such as a motor, is required, as an urging force generating device, to urge the pushing member. Thus, the total production cost of the sheet feeder can be reduced.